The present invention generally relates to dry refractory mix compositions and the method of preparing the same, and more particularly, to a single package phosphate bondable dry refractory mix composition, and a high intensity mixing process for preparing the mix.
The term "refractory specialty" has often been used to refer to a broad class of unconsolidated refractories, i.e. not in preformed shapes such as molded brick. Refractory specialties are typically utilized in monolithic refractory constructions wherein the refractory is installed in situ and forms an integral jointless structure. Many refractory specialties are popularly produced, packaged, and shipped as mixes--blends of materials proportioned in a definite manner. These mixes may be produced and shipped in a substantially wet or plastic condition for use without further treatment, in a dry state requiring liquid tempering and mixing, or as a two package (one dry, one wet) system requiring intermixing of the package components with or without the addition of a liquid prior to the use of the mix. Thus, refractory specialty mixes used for monolithic refractory construction may be further distinguished and classified, in either of two ways, by the condition in which they are shipped, e.g., plastic, dry, and wet, or moreover, by the techniques by which they are applied, e.g., ramming, gunning, casting and troweling, or both. Wet refractories are substantially wet to the touch and, conversely, dry refractories are substantially dry to the touch.
Plastic refractories are refractory materials, generally tempered with water into a stiff plastic condition having a desired consistency, that can be extruded and that have suitable workability for use in forming a monolithic structure without further preparation. Plastic refractories are often rammed into place.
Ramming mixes, by definition, consist essentially of ground and sized refractory aggregates, with amounts of other materials added to promote workability and bonding, that cannot be extruded but have suitable properties to permit ramming into place to form a monolithic structure. Hence, ramming mixes are usually shipped in a wet state and further liquid addition is not required for the application technique.
A castable, in contrast, is defined as a combination of refractory grain and suitable bonding agent that, after the addition of a proper liquid, is generally poured into place to form a refractory shape or structure which becomes rigid due to chemical action. Castables are generally cast or gunned into place.
Refractory mix compositions which utilize high percentages of various inert refractory aggregates, especially alumina, and which include phosphoric acid or phosphates are known in the art. When phosphoric acid or acid phosphate salts are used to generate a chemical bond premature hardening and loss of workability is often encountered and limits the shelf life of the mixes. The premature uncontrolled presetting of these phosphate bondable mixes is believed to be caused by a series of complex chemical reactions between the phosphate ingredients and the alumina bearing materials in the mix.
Presently, there are phosphate bondable refractory mix products marketed as a two package (each having multiple ingredients) system. A typical two package product includes a package having damp mix ingredients containing tabular alumina and an equilibrium mixture of phosphoric based acid, and a package of dry mix ingredients containing tabular alumina, calcined alumina, hydrated alumina and a calcium aluminate cement. Blending of the ingredients of the two packages with the addition of water to achieve a desired consistency initiates setting of the mixture. Because the damp and dry packages cannot be intermixed until a short time before the mix is to be applied, each package must be separately produced, packed and maintained. The shelf life of the damp package, moreover, is limited in time due to reactions of its constituent acid and alumina. Such reactions are accelerated if the damp component package is contaminated, poorly sealed or exposed to excessive heat. The limited shelf life of the damp component package can result in warehousing or production scheduling problems or both. Use of a two package mix also exposes the placement operation to the possibility of errors, e.g., in weighing and mixing the components, which can result in an inferior product and wastage, and increased preparation time of the user. Clearly, a single package mix having an extended shelf life can lead to improved economies.
Techniques for extending the shelf life of single or two package phosphate bondable preparations generally encompass use of a sequestering agent or inhibitor to retard premature reaction between the alumina and phosphoric acid. U.S. Pat. No. 3,622,360, for example, discloses an alumina-phosphoric acid ramming mix composition. The ramming mix of U.S. Pat. No. 3,622,360 is described as having an extended shelf life and prolonged workability specifically due to the addition of an inhibitor selected from the group consisting of nitrilotriacetic acid and ethylenediaminetetracetic acid which promote the age retardation of the mix. The ramming mix described is tempered with water to achieve a desired plasticity.
U.S. Pat. No. 3,197,315 discloses the formulation and use of a typical high alumina content wet refractory composition, packed in suitable paper containers for shipment, utilizing 85% phosphoric acid. The alumina particles, as an additional requirement, are precoated with heated fatty acids such as palmitic or stearic acids or mixtures thereof to produce a lubricated, free-flowing gunning composition. The composition disclosed therein also contains amounts of boric acid used as a binder in conjunction with the phosphoric acid. The addition of small amounts of boric acid in such a composition is known to retard alumina-phosphoric acid reactions. The composition is prepared by heating the fatty acid mixture components to above the melting point of the fatty acid mixture which is added to a premixed batch of the remaining ingredients for intermixing of the entire blend.
U.S. Pat. No. 3,303,034 teaches a plastic refractory mixture with phosphoric acid and aluminous material from the group consisting of tabular alumina, bauxite, kyanite and diaspore that is disclosed as suitable for installation by ramming techniques after storage periods of up to six months. The improved storage shelf life is attributed to the presence of bentonite as a plasticizer which is less reactive with the phosphoric acid than the previously used plasticizers.
U.S. Pat. No. 2,852,401 teaches high temperature phosphate bondable refractory compositions in the form of unconsolidated particles that are substantially dry to the touch. The compositions of U.S. Pat. No. 2,852,401 are formed by incorporating phosphoric acid in a refractory batch having a major proportion of refractory aggregate and a minor proportion of aluminous material that is chemically reactive with phosphoric acid at room temperature, followed by drying of the mixture at a temperature not in excess of about 125.degree. F. Hence, the refractory aggregate must be sufficiently inert that it does not, upon the addition of the acid, produce sufficient heat of reaction to bring the batch temperature above the critical 125.degree. F. upon which the desirable properties of the disclosed compositions depend.
Prior art techniques for preparation of the two component mixes typically rely on a thorough mixing of each component separately. At the time of use, the components are intermixed with water and again subjected to further mixing.
Thus, the development of a dry alumina based, phosphate bondable single package refractory mix composition which exhibits extended shelf life, which does not require an inhibitor is highly desirable. Moreover, a simplified process of preparing such a composition which eliminates necessary prior art process steps for preparing the mix such as drying or precoating aluminous materials with combinations of fatty acid under controlled conditions offers further attraction.